Grandidierite

specimen number:
2072374

location:
Tranomaro, Amboasary, Anosy, Tulear, Madagascar

description:
An outstanding example for this species with attractive bluish bladed crystals of translucent in measuring to 3.0 cm in length. In good condition.

The Tranomaro area, located within the Amboasary District of the Anosy Region in southern Madagascar, is the primary global source for the rare borosilicate mineral grandidierite. Geologically, the deposit is situated within the high-grade metamorphic rocks of the Androyan granulite facies, specifically occurring in aluminous pegmatites and gneissic rocks where boron-rich fluids interacted with host rocks under extreme pressure and temperature. Grandidierite is prized for its distinct bluish-green to sea-foam green color and strong pleochroism, typically forming as elongated, translucent to transparent crystals or massive aggregates. Historically, while the mineral was first discovered in 1902 by Alfred Lacroix near Andrahomana, the specific Tranomaro deposits gained international prominence for producing gem-quality material starting around 2014 and 2015. Mining and quarrying in the region are predominantly artisanal, with local miners utilizing hand tools to excavate the rugged terrain and recover specimens from deep within the weathered host rock. As of 2026, Tranomaro remains the most significant locality for both collector-grade crystal specimens and rare faceted gemstones of this mineral species.

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